Initial impedance decrease as an indicator of good catheter contact: Insights from radiofrequency ablation with force sensing catheters Tobias Reichlin, MD,*† Sven Knecht, PhD,† Christopher Lane, MD,* Michael Kühne, MD,† Eyal Nof, MD,*‡ Nagesh Chopra, MD,* Thomas M. Tadros, MD, MPH,* Vivek Y. Reddy, MD,§ Beat Schaer, MD,† Roy M. John, MD, PhD, FHRS,* Stefan Osswald, MD,† William G. Stevenson, MD, FHRS,* Christian Sticherling, MD,† Gregory F. Michaud, MD, FHRS* From the *Cardiovascular Division, Brigham and Women’s Hospital, Boston, Massachusetts, †University Hospital, Basel, Switzerland, ‡Leviev Heart Center, Sheba Medical Center, Tel Hashomer, Israel, and § Helmsley Electrophysiology Center, The Cardiovascular Institute, Mount Sinai School of Medicine, New York, New York. BACKGROUND Good catheter–tissue contact force (CF) is critical for transmural and durable lesion formation during radiofrequency (RF) ablation but is difficult to assess in clinical practice. Tissue heating during RF application results in an impedance decrease at the catheter tip. OBJECTIVE The purpose of this study was to correlate achieved CF and initial impedance decreases during atrial fibrillation (AF) ablation. METHODS We correlated achieved CF and initial impedance decreases in patients undergoing ablation for AF with two novel open-irrigated CF-sensing RF catheters (Biosense Webster SmartTouch, n ¼ 647 RF applications; and Endosense TactiCath, n ¼ 637 RF applications). We then compared those impedance decreases to 691 RF applications with a standard open-irrigated RF catheter (Biosense Webster ThermoCool). RESULTS When RF applications with the CF-sensing catheters were analyzed according to an achieved average CF o5 g, 5–10 g, 10–20 g, and 420 g, the initial impedance decreases during ablation were larger with greater CF. Corresponding median values at 20 seconds were 5 Ω (interquartile range [IQR] 2–7), 8 Ω (4–11), 10 Ω (7–16), and 14 Ω (10–19) with the SmartTouch and n/a, 4 Ω (0–10), 8 Ω

(5–12), and 13 Ω (8–18) with the TactiCath (P o.001 between categories for both catheters). When RF applications with the SmartTouch (CF-sensing catheter, median achieved CF 12 g) and ThermoCool (standard catheter) were compared, the initial impedance decrease was significantly greater in the CF-sensing group with median decreases of 10 Ω (6–14 Ω) vs 5 Ω (2–10 Ω) at 20 seconds (P o.001 between catheters). CONCLUSION The initial impedance decrease during RF applications in AF ablations is larger when greater catheter contact is achieved. Monitoring of the initial impedance decrease is a widely available indicator of catheter contact and may help to improve formation of durable ablation lesions. KEYWORDS Contact force sensing catheter; Impedance decrease; Atrial fibrillation ablation ABBREVIATIONS AF ¼ atrial fibrillation; CF ¼ contact force; EGM ¼ electrogram; IQR ¼ interquartile range; PV ¼ pulmonary vein; PVI ¼ pulmonary vein isolation; RF ¼ radiofrequency (Heart Rhythm 2014;11:194–201) I 2014 Heart Rhythm Society. All rights reserved.

Introduction Dr. Reichlin has received research grants from the Swiss National Science Foundation (PASMP3-136995), the Swiss Heart Foundation, the Professor Max Cloëtta Foundation, and the Uniscientia Foundation Vaduz. Dr. Reddy serves as a consultant to and receives grant support from St. Jude Medical, Biosense Webster, and Endosense. Dr. John has performed industry-sponsored research for Biosense Webster and Thermedical and has received consultant and speaking honoraria from St. Jude Medical. Dr. Stevenson is a holder of a U.S. patent for needle ablation consigned to the Brigham and Women’s Hospital. Dr. Michaud has received research support from Boston Scientific and serves as a consultant to Boston Scientific, Medtronic, and St. Jude Medical. Address reprint requests and correspondence: Dr. Tobias Reichlin, Division of Cardiology, University Hospital Basel, Petersgraben 4, CH-4031 Basel, Switzerland. E-mail address: [email protected].

1547-5271/$-see front matter B 2014 Heart Rhythm Society. All rights reserved.

Radiofrequency (RF) catheter ablation therapy by means of pulmonary vein isolation (PVI) has emerged as an effective treatment of drug-refractory atrial fibrillation (AF).1 Recurrence of AF after a single procedure has been reported to range from 15% to 60% after 1 year and from 50% to 70% after 5 years depending on patient selection.2–4 Recovery of pulmonary vein (PV) conduction following an initially successful PVI is one of several important factors leading to AF recurrence.5,6 Good catheter–tissue contact force (CF) is critical for transmural lesion formation7–9 and to avoid conduction recovery.10 Indirect parameters used to assess CF are tactile http://dx.doi.org/10.1016/j.hrthm.2013.10.048

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feedback, local electrogram (EGM) amplitude and morphology, imaging of the catheter tip with fluoroscopy or intracardiac echocardiography, and catheter tip impedance monitoring. For impedance monitoring more specifically, experimental data suggest that increased tissue contact is associated with a larger initial impedance drop during an RF application, whereas ambiguous observations were found for the starting impedance.9,11–13 Recently, RF ablation catheters with a CF sensor at the distal tip have allowed continuous measurement and real-time display of the CF between the catheter tip and the tissue.14 A previous pilot study using the TactiCath CF-sensing catheter (Endosense SA, Geneva, Switzerland) found a moderate correlation between achieved CF and impedance decreases during RF ablation,15 but quantitative data comparing initial impedance decreases and catheter CF in humans using the SmartTouch CF-sensing catheter (Biosense Webster, Diamond Bar, CA, USA) are lacking. The aim of our study was to assess the association between the initial impedance decrease and the average CF achieved during RF applications when using a CF-sensing catheter. In addition, we compared the initial impedance decreases during RF applications with a CF-sensing catheter to those with a standard catheter where the CF was unknown.

Methods Study population A total of 49 patients undergoing catheter ablation for paroxysmal AF with PVI were retrospectively analyzed. Ablation was performed using either one of two novel openirrigated CF-sensing RF catheters (SmartTouch, n ¼ 25 patients at University Hospital Basel; or TactiCath, n ¼ 12 patients at Brigham and Women’s Hospital Boston) or a standard open-irrigated RF catheter (Biosense Webster NaviStar ThermoCool catheter, n ¼ 12 patients at Brigham and Women’s Hospital Boston). The study protocol was approved by the Brigham and Women’s Hospital Human Subject Protection Committee and the Basel Ethics Committee. A total of 2941 RF applications were performed in the 49 patients. Stable contact of the catheter was defined as a visually stable position and a uniform force pattern over a minimum of 20 seconds after the start of ablation. Therefore, a total of n ¼ 961 RF applications (33%) with a duration o20 seconds or an unstable catheter position were excluded from analysis (669/1316 [51%] in the SmartTouch catheter group, 208/850 [24%] in the TactiCath group, and 84/775 [11%] in the standard catheter group), as were five RF applications with missing CF information in the TactiCath group. This left 1975 RF applications for analysis, of which 647 were performed with the SmartTouch catheter, 637 with the TactiCath catheter, and 691 with the standard ablation catheter.

Ablation catheters The SmartTouch catheter is a steerable 3.5-mm six-hole open-irrigated tip ablation catheter. CF measurement is performed via recording of microdeformation of a precision

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spring connecting the tip and the shaft of the catheter.16 The CF is measured every 50 ms and displayed continuously in real time in grams. The TactiCath catheter is a steerable 3.5-mm-tip openirrigation RF ablation catheter. CF measurement is obtained using a triaxial fiberoptic cable inside the catheter. The CF is measured every 100 ms and displayed continuously in grams.14 Ablation in the standard catheter group was performed using the NaviStar ThermoCool catheter. This is a steerable catheter that has the same 3.5-mm six-hole open-irrigated tip design as the SmartTouch catheter.17 Flow rate during ablation was 17 mL/min in all three groups.

Electrophysiologic study and catheter ablation A nonfluoroscopic three-dimensional mapping system was used in each case: the CARTO 3 system (Biosense Webster) in the SmartTouch and ThermoCool catheter groups and the EnSite NavX system (St. Jude Medical, St. Paul, MN, USA) in the TactiCath catheter group. Surface and intracardiac ECGs were digitally recorded and stored (Prucka CardioLab EP System, GE Healthcare, Waukesha, WI, USA, in the TactiCath and ThermoCool groups, and Sensis EP, Siemens, Erlangen, Germany, in the SmartTouch group). A double transseptal approach was used. A 10- or 20-pole circumferential PV mapping catheter was positioned at the ostium of one of the ipsilateral PVs, usually the superior vein. A long steerable sheath (Agilis 8.5Fr steerable catheter introducer, St. Jude Medical was used at the discretion of the operator. Ablation lesions were delivered using the same RF generator (Stockert, Freiburg, Germany) in a power-controlled mode. Power delivery was set at 30 W in the left atrium and was reduced to 15–25 W for ablation on the posterior wall adjacent to the esophagus. Power settings higher than 30 W were used in o1% of lesions. Power settings were not changed during RF delivery, and changes were made only between RF applications. The lesion set consisted of two wide circumferential antral ablation lines around ipsilateral PV pairs. Lesions were applied point by point, and dragging was limited as much as possible. A cavotricuspid isthmus line was added in the right atrium if typical atrial flutter had been documented previously or occurred during the procedure. The procedural end-point was bidirectional conduction block into and out of PV pairs as assessed by circular mapping catheter and unexcitability of the ablation line as assessed by bipolar pacing at an output of 10 mA/2.0 ms.18 Further linear ablation or ablation of complex fractionated atrial electrograms was avoided in general and performed only exceptionally if AF or atrial flutter occurred during the procedure and would not terminate after achieving PVI. During left atrial ablation, heparin was administered intravenously to maintain an activated clotting time Z350 seconds.

Measurement of impedance and CF during RF application Values of impedance, power, and temperature were continuously recorded with a sampling frequency of every 100 ms

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Figure 1 Impedance plot as recorded by the generic electrophysiology laboratory recording system during each radiofrequency application. Data were analyzed for starting impedance levels as well as for absolute and relative decreases within 5, 10, 15, and 20 seconds.

in the electrophysiology recording system (Prucka CardioLab EP System) or mapping system (CARTO 3) and were later exported for data processing. The analysis focused on the changes in impedance within the first 20 seconds of the RF applications in order to avoid effects introduced by unintentional dragging of the catheter. From the exported data, impedance was measured at the beginning of the ablation as well as after 5, 10, 15, and 20 seconds to calculate the absolute and relative impedance changes within the first 20 seconds (Figure 1). For the 1289 RF applications performed with the CF-sensing catheters, the real-time CF values were available to the operators (GM, MK, CS) with both CF-sensing catheters. CF was recorded every 50 and 100 ms by the CF-sensing catheter systems, respectively (CARTO 3 and TactiCath). Given that real-time CF shows some fluctuations with the patient’s breathing pattern and the contractility of the heart even with a stable catheter position, mean CF over the first 20 seconds was calculated offline from all recorded CF values during the first 20 seconds. For analysis according to the CF achieved per RF application, categories of o5 g, 5–10 g, 10–20 g, and 420 g were used based on the recently published association of arrhythmia recurrence for patients in these categories.10 Prospective validation of the identified cutoff values to distinguish between good and poor contact was performed in an additional set of 119 ablation points applied in six patients with the SmartTouch catheter.

Statistical analysis Continuous variables are presented as mean ⫾ SD or as median with interquartile range (IQR). Categorical variables are presented as numbers and percentages. Continuous variables were compared with the Mann-Whitney U test and categorical variables using the Pearson χ2 test. Correlations among continuous variables were assessed using the Spearman rank correlation coefficient. Optimal cutoff values Table 1

of impedance decreases at 10 seconds for prediction of CF Z20 g were derived from receiver operating characteristic curves as described by Youden.19 All hypothesis testing was two tailed, and P o.05 was considered significant. All statistical analyses were performed using SPSS for Windows 19.0 (SPSS Inc, Chicago, IL, USA).

Results Baseline characteristics of patients The baseline characteristics of all 49 patients are listed in Table 1. Patients in the three catheter groups were very similar with regard to clinical characteristics, including age, gender, ejection fraction, and left atrial size.

Achieved CF in the CF-sensing catheter groups In the SmartTouch CF-sensing catheter group, the median achieved CF during RF applications was 12 g (IQR 8–17). In more detail, the median achieved CF per RF application was o5 g in 65 applications (10%), 5–10 g in 196 applications (30%), 10–20 g in 283 applications (44%), and 420 g in 103 applications (16%). In the Endosense CF-sensing catheter group, the median achieved CF was 27 g (IQR 22–34). In more detail, the average CF achieved per RF application was 5–10 g in 15 applications (3%), 10–20 g in 111 applications (17%), and 420 g in 511 applications (80%). No RF applications were performed with CF o5 g in this group.

Comparison of starting impedances and achieved CF No correlation was observed between starting impedance values and achieved CF in either the SmartTouch group (r ¼ 0.05, P ¼ .40) or the TactiCath group (r ¼ 0.00, P ¼ .97). When analyzed according to the CF categories described earlier, median starting impedances were 123 Ω (IQR 118–136), 124 Ω (118–132), 125 Ω (117–133), and 129 Ω (119–139) with the SmartTouch (P ¼ .07 for comparisons between categories; Figure 2A), and n/a, 155 Ω (144–181), 144 Ω (130–190), and 155 Ω (137–198) with the TactiCath (P ¼ .11 for comparisons between categories; Figure 2B).

Comparison of initial absolute impedance decrease and achieved CF A significant correlation was found between the initial impedance decrease and the achieved CF, with initial impedance decreases being larger when greater CF was achieved (SmartTouch r ¼ 0.39 at 10 seconds, 0.45 at

Baseline and procedural characteristics of the patients

Age (years) Male gender [no.(%)] Left ventricular ejection fraction (%) Left atrial diameter (mm) CF ¼ contact force.

SmartTouch CF-sensing catheter (n ¼ 25)

TactiCath CF-sensing catheter (n ¼ 12)

ThermoCool standard catheter (n ¼ 12)

58 17 60 41

60 (52–67) 9 (75) 58 (53–66) 37 (30–45)

61 (53–69) 7 (58) 58 (55–62) 35 (31–42)

(52–65) (68) (49–60) (36–46)

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197 SmartTouch CaSDN r = 0.45

Tacticath Catheter r = 0.34

Figure 3 Scattergram showing the association between the achieved average contact force and the initial impedance decrease after 20 seconds for the SmartTouch catheter (A) and the TactiCath catheter (B).

Comparison of initial relative impedance decrease and achieved CF

Median impedance decrease (Ω)

20 seconds; TactiCath r ¼ 0.31 at 10 seconds, 0.34 at 20 seconds, all P o.01; Figures 3A and 3B). For the SmartTouch catheter, median impedance decreases for applications with achieved CF of o5 g, 5–10 g, 10–20 g, and 420 g were 4.1 Ω (1.9–6.7), 7.0 Ω (4.2–9.8), 8.9 Ω (6.0–11.9), and 11.4 Ω (8.115.0), respectively, at 10 seconds; and 4.7 Ω (1.8–7.3), 8.0 Ω (4.2–11.3), 10.3 Ω (7.2–15.8), and 13.7 Ω (10.2–18.8), respectively, at 20 seconds (P o.001 for comparisons between categories; Figures 4A and 5A). For the TactiCath catheter, median impedance decreases for the four categories were n/a, 3.3 Ω (-0.9–11.1), 7.2 Ω (4.1–10.5), and 11.2 Ω (7.0–15.7) at 10 seconds; and n/a, 3.9 Ω (-0.3–9.6), 7.8 Ω (4.6–11.8), and 13.0 Ω (7.5–18.3) at 20 seconds (P o.001 for comparisons between categories; Figures 4B and 5B).

Median impedance decrease (Ω)

Figure 2 Starting impedance levels according to the achieved contact force. A: Starting impedance levels for radiofrequency applications in the SmartTouch contact force (CF)-sensing catheter group according to the achieved average force per radiofrequency application for the four groups o5 g, 5–10 g, 10–20 g, and 420 g. B: Starting impedance levels for radiofrequency applications in the TactiCath CF-sensing catheter group according to the achieved average force per radiofrequency application for the three groups 5–10 g, 10–20 g, and 420 g. Boxes represent interquartile ranges. Whiskers represent ranges.

Similarly, relative impedance decreases were significantly associated with larger CF (SmartTouch r ¼ 0.39 at 10 seconds, 0.46 at 20 seconds; TactiCath r ¼ 0.34 at 10 seconds, 0.34 at 20 seconds, all P o.01). For the SmartTouch catheter, median relative impedance decreases for applications with achieved CF of o5 g, 5–10

p < 0.001

p < 0.001

Figure 4 Median initial impedance decreases during ablation according to the achieved contact force (CF). Median values of absolute impedance decreases in ohms (Ω) within the first 5, 10, 15, and 20 seconds during radiofrequency applications for radiofrequency applications performed with the SmartTouch CF-sensing catheter (A) and the TactiCath CF-sensing catheter (B) according to the achieved CF for the groups o5 g, 5–10 g, 10–20 g, and 420 g.

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Heart Rhythm, Vol 11, No 2, February 2014 No association between power used during RF application and initial absolute or relative impedance decrease was found (all r o0.1, all P 4.05).

Discrimination of good and poor catheter CF based on initial impedance decrease Using receiver operating characteristic curve analysis, the best cutoff values for discrimination of good CF (420 g) vs poor contact CF (o20 g) based on the impedance decrease after 10 seconds were assessed. The best cutoff values for this discrimination ware an impedance decrease by 10 seconds of 8.9 Ω with the SmartTouch catheter and of 8.4 Ω with the TactiCath CF-sensing catheter. Applying these values to our data resulted in sensitivity and specificity of 70% and 62% with the SmartTouch catheter, respectively, and 67% and 63%, respectively, with the TactiCath CFsensing catheter. Negative and positive predictive values were 92% and 26% with the SmartTouch catheter, respectively, and 32% and 88%, respectively, with the TactiCath CF-sensing catheter. The best cutoff values for a relative impedance change after 10 seconds were 8.2% for the SmartTouch catheter and 6.5% for the TactiCath catheter. These cutoff values resulted in sensitivity and specificity of 71% and 61% for the SmartTouch catheter, respectively, and 57% and 70% for the TactiCath catheter, respectively. Negative and positive predictive values were 91% and 29% for the SmartTouch catheter, respectively, and 29% and 89% for the TactiCath catheter, respectively. The cutoff values obtained for the SmartTouch catheter from this retrospective analysis were then prospectively validated in an additional set of 119 ablation points performed in six patients. An absolute impedance decrease of at least 8.9 Ω after 10 seconds predicted CF 420 g with sensitivity and specificity of 91% and 54%, respectively, whereas negative and positive predictive values were 96% and 30%, respectively. Use of a relative impedance decrease of at least 8.2% after 10 seconds predicted CF 420 g with sensitivity and specificity of 71% and 73%, respectively, whereas negative and positive predictive values were 92% and 37%, respectively. Figure 5 Initial impedance decreases during ablation according to the achieved contact force (CF). Initial impedance decreases after 10 seconds and 20 seconds during radiofrequency application are displayed according to the achieved average force for the groups o5 g, 5–10 g, 10–20 g, and 420 g. Boxes represent interquartile ranges. Whiskers represent ranges.

Comparison of starting impedances and initial impedance decreases between the SmartTouch CF-sensing and the standard catheter groups

g, 10–20 g, and 420 g were 3.1% (1.5–5.2), 5.6% (3.5–7.8), 7.2% (4.9–9.3), and 8.9% (6.2–11.7), respectively, at 10 seconds; and 3.8% (1.7–6.0), 6.6% (3.3–9.0), 8.6% (6.0–12.0), and 11.1% (8.5–14.1), respectively, at 20 seconds (P o.001 for comparisons between categories). For the TactiCath catheter, median impedance decreases for the four categories were n/a, 2.3% (-0.6–7.7), 4.6% (2.7–7.4), and 7.2% (4.2–9.9) at 10 seconds; and n/a, 2.5% (-0.3–8.0), 5.4% (3.1–8.1), and 8.4% (4.6–11.6) at 20 seconds (P o.001 for comparisons between categories).

The starting impedance was higher in patients in the standard catheter group compared to the SmartTouch CF-sensing catheter group (Figure 6A). Median starting impedances were 134 Ω (125–143 Ω) in the standard catheter group compared to 125 Ω (118–134 Ω) in the SmartTouch CFsensing catheter group (P o.001 for comparison between catheters). The initial impedance decreases were significantly larger in the CF-sensing group compared to the standard catheter group (Figure 6B). Median impedance decreases in the standard catheter group compared to the CF-sensing catheter

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Similarly, median relative impedance decreases in the standard catheter group compared to the CF-sensing catheter group were 3.6% (1.7–6.0) vs 6.5% (3.9–9.1) after 10 seconds and 4.1% (1.5–7.0) vs 7.7% (4.6–11.3) after 20 seconds (P o.001 for comparisons between catheters).

Discussion

Standard RF Catheter

SmartTouch CF-sensing RF catheter

p < 0.001

Standard RF Catheter

SmartTouch CF-sensing RF catheter

Figure 6 Comparison of starting impedance levels and initial impedance decreases during ablation between the standard radiofrequency (RF) catheter and the SmartTouch contact force (CF)-sensing catheter. A: Starting impedance levels for radiofrequency applications in the standard RF catheter group and the SmartTouch CF-sensing catheter group. B: Median values of absolute impedance decreases in ohms (Ω) within the first 5, 10, 15, and 20 seconds during RF applications for RF applications performed with the standard catheter and the SmartTouch CF-sensing catheter. C: Initial impedance decreases after 10 and 20 seconds during RF application for the standard catheter group and the SmartTouch CF-sensing catheter group. Boxes represent interquartile ranges. Whiskers represent ranges.

group were 4.8 Ω (2.2–8.1) vs 8.2 (4.9–11.7) after 10 seconds and 5.4 Ω (2.0–9.6) vs 9.5 Ω (5.7–14.3) after 20 seconds (P o.001 for comparisons between catheters; Figure 6C).

This study assesses the association of initial impedance decrease with CF during RF applications in AF ablation procedures with two novel CF-sensing catheters and compares impedance decreases with a CF-sensing catheter vs a standard RF catheter. We report three major findings. First, when analyzed according to the median CF achieved with a CF-sensing catheter, the initial impedance decrease during an RF application is significantly larger when greater CF is achieved, and the findings are very similar for both CF-sensing catheters. Second, the median initial impedance decrease is significantly larger when ablation is performed with a CF-sensing RF catheter than with a standard RF catheter with a very similar design that does not display CF. Third, a median impedance decrease of around 8–10 Ω or 6%–8% within the first 10 seconds may emerge as a useful indicator of good catheter contact. Our findings confirm experimental studies demonstrating greater initial impedance decreases when greater catheter– tissue CF was applied.9,11,13,20 In addition, clinical studies found that RF applications with an initial impedance decrease seemed to occur more frequently with “firm” rather than “poor” contact and led to more effective lesions as indicated by interruption of conduction.12,15,21 With current RF irrigated catheter technologies, PVI can be achieved acutely in almost all patients; however, many patients have recurrence of AF. Recovery of PV conduction appears to be the most important factor leading to AF recurrence.5,6 Therefore, any measure to safely improve durability of ablation lesions is of great clinical interest, and the development of CF-sensing catheters is promising from that perspective.14 Experimental studies convincingly demonstrate a strong relationship between applied CF and resulting lesion size.8,9 Pilot studies using a CF-sensing catheter during AF ablations have demonstrated clinical evidence of the relationship between achieved catheter tip CF during ablation and the frequency of AF recurrence after 1 year.10 More specifically, Reddy et al10 found a recurrence rate of 100% after 1 year if an average CF o10 g was achieved during the ablation. On the other hand, 80% of the patients with an achieved average CF 420 g were free of AF recurrence after 1 year.10

Clinical significance Unfortunately, CF-sensing catheters are not routinely available in clinical practice in some parts of the world, including the United States. In that context, our findings may be particularly useful with currently available standard openirrigated RF catheters, for which impedance monitoring is widely available. First, catheter ablation operators will obtain

200 a good idea of what relative CF they achieve by calculating the median impedance decrease within the first 20 seconds over multiple lesions and locations. Operators should aim for a median impedance decrease of 8 Ω (or a relative decrease of 6%–8%) within the first 10 seconds. It is noteworthy that the negative predictive value was particularly high in this dataset when lower CF was achieved (median CF 12 g with the SmartTouch catheter with negative predictive value 90% as opposed to median CF 27 g with the TactiCath catheter with negative predictive value 30%). This suggests that operators who generally achieve lower CF might have the greatest benefit from using the initial impedance decrease as an additional indirect assessment of catheter contact. High CF operators will see far fewer lesions with small impedance decreases and will need to use other parameters such as tactile feedback, fluoroscopy, and local EGM signal reduction and morphology changes during RF application to determine whether the CF actually is low.

Impedance decrease with standard catheter The direct comparison of the SmartTouch CF-sensing catheter with the Navistar ThermoCool standard catheter, both of which are very similar in design, showed a significantly larger initial impedance decrease with the SmartTouch catheter, even though an average CF of only 12 g was achieved overall. Unfortunately, this finding suggests that poor contact is achieved between the catheter tip and tissue interface with tactile feedback and other indirect parameters alone.

Impedance decrease and individual lesion assessment Although we observed an overall association between achieved CF and initial impedance decrease, there was significant overlap of ranges, as has been observed in a similar study.15 For some of the RF applications, good contact resulted in only a limited initial impedance decrease, whereas in other RF applications rather poor contact showed more pronounced impedance decreases. Monitoring of the initial impedance decrease is a reasonable, albeit not perfect, surrogate for measuring catheter–tissue CF, as demonstrated by the limited number of lesions studied prospectively, where the negative predictive value remained 490% for good contact. It is important from a safety standpoint to emphasize that this information has to be applied in conjunction with other parameters used to indirectly assess catheter contact, such as tactile feedback, fluoroscopy, and local EGM signal reduction and morphology changes during RF application. Also, lesions were delivered with limited power (r30 Ω) in all locations and with even further reduced power (r20 W) and duration (r20 seconds) when the catheter tip was on the posterior wall of the left atrium adjacent to the esophagus. Prospective analysis of the correlation between impedance decrease and individual lesion assessment will be necessary to ensure optimal safety and efficacy.

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Study limitations Our study has several limitations. First, in contrast to the real-time information provided by the CF-sensing catheter, the information on impedance decreases is available only after beginning an RF application. Even prematurely terminated RF applications with insufficient CF have the potential to generate edema-producing lesions that may inhibit the ability to deliver subsequent lesions. In addition, an impedance monitoring approach lacks the potential of CF-sensing catheters to enhance the safety of ablation procedures by avoiding very high contact pressures at the catheter tip during mapping. Second, not only the CF but also the orientation of the tip relative to the wall is likely to influence the resulting impedance decrease during an RF application, and we had no specific information on catheter orientation available in our study. Third, a recent study has shown a large variation of achieved CF in different locations of the left atrium.22 It is likely that this also translates into anatomic differences in impedance decreases, but we do not have anatomic data on the distribution of impedance decreases in our cohort. Fourth, excessive impedance decreases have been shown to precede steam pops and coagulum formation, particularly at higher power settings.9,21,23,24 Limiting power to 30 W in the left atrium in general and to 15–25 W on the posterior wall, as we did in this study, may be important to prevent the occurrence of steam pops despite large impedance decreases. Based on data from the limited number of patients studied, we cannot deduce a safe combination of power and impedance decreases.

Conclusion The initial impedance decrease during an RF application is significantly larger when greater CF is achieved. The smaller impedance decreases observed with a standard RF catheter indicate that poor contact is often achieved with tactile feedback alone, which likely contributes to poor durability of PVI and frequent recurrences after AF ablations. Our data suggest that, in the absence of a CF-sensing catheter, monitoring of the median initial impedance decrease may be used as a reasonable additional indicator of good catheter contact, which can be used in conjunction with several other indirect surrogates of catheter contact such as tactile feedback, fluoroscopy, and the local EGM. Further studies are needed to assess the relative contributions of CF-sensing and impedance monitoring in safely producing durable PVI and improving AF ablation outcomes.

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Initial impedance decrease as an indicator of good catheter contact: insights from radiofrequency ablation with force sensing catheters.

Good catheter-tissue contact force (CF) is critical for transmural and durable lesion formation during radiofrequency (RF) ablation but is difficult t...
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